Geophysical electromagnetic (EM) techniques may be effective in the determination of the electrical conductivity of soils, rocks and other conductive material at depths from the surface up to about three kilometers. Conductivity distribution at such depths is of great interest to those involved in mapping base metal and uranium deposits, aquifers and other geological formations.
Geophysical EM methods involve measurement of time-varying magnetic fields near the earth's surface produced by a primary magnetic field and modeling of the ground conductivity distributions. These magnetic fields are generated either by a periodic current applied to a transmitter, or by naturally occurring electromagnetic fields originating mainly from lightning in the earth's atmosphere. EM fields can have a characteristic ground penetration depth proportional to the inverse of the square-root of both ground conductivity and frequency.
In known methods, the magnetic field signal is measured using either a receiver coil system (which can measure up to three orthogonal components of the magnetic field time-derivative dB/dt), or a magnetometer (which measures the magnetic field B). The received analog signal is then amplified, filtered, and digitized by a high-resolution high-speed analog-to-digital converter (ADC), and the data may be stored along with the positioning information obtained from a Global Positioning System (GPS). Data post-processing involves electrical and physical modeling of the ground to generate the geophysical conductivity contour maps.
Existing geophysical surveying methods typically require high signal-to-noise ratio (SNR), high conductivity discrimination, and high spatial resolution both laterally and in depth. A preferred EM method, known as a time-domain electromagnetic (TDEM) measurement, depends on ground conductivity and the desired probing depth. TDEM measurements can probe targets at depths up to 1 km in the conductivity range from about 10 mS/m to 10 S/m.
Other known techniques, such as Frequency-Domain Electromagnetic (FDEM) measurements, can provide well calibrated measurements at depths up to about 100 m in the conductivity range 1 mS/m to 1 kS/m. Audio Frequency Magnetic (AFMAG) measurements can measure targets at depths up to 3 km in the conductivity range 0.1 mS/m to 1 S/m with high conductivity contrast.
Furthermore, existing EM systems encompass both ground-based and airborne measurements. Airborne measurements are collected through the use of airplanes and helicopters. Airborne methods are preferred for large area surveys and may be used for exploration of conductive ore bodies buried in resistive bedrock, geological mapping, hydrogeology, and environmental monitoring. Known airborne electromagnetic (AEM) systems function so that the data is acquired while the airplane or helicopter flies at nearly constant speed (e.g. up to 75 m/s or 30 m/s, respectively) along nearly-parallel equally-spaced lines (e.g. 50 m to 200 m) at close to constant height above ground (e.g. about 120 m or 30 m, respectively). Measurements are taken at regular intervals, typically in the range 1 m up to 100 m.
Fixed-wing airplane AEM systems specifically can carry large transmitter coils and are capable of deeper investigation of discrete conductors, such as base metal and uranium deposits, while helicopter frequency-domain electromagnetic systems are effective in near-surface mapping with high near-surface resolution, although limited depth penetration. Over the last decade, the key drive in geophysical surveying has been to develop helicopter-mounted time-domain electromagnetic (HTEM) systems.
An additional feature of known EM measurements is that they can be achieved either in the frequency domain or time domain. In FDEM measurements, the transmitter coil continuously transmits an electromagnetic signal at fixed multiple frequencies, while the receiver coil measures the signal continuously over time. The measured quantities are signal amplitude and phase as a function of frequency, or equivalently, the in-phase and in-quadrature amplitudes as a function of frequency. In these measurements, the signal sensitivity is reduced with increasing conductivity, thus reducing the conductivity contrast mapping.
In the course of collecting time TDEM measurements by known methods, a pulse of current is applied to the transmitter coil during an on-period and switched off during the off-period, typically at a repetition rate equal to an odd multiple of half of the local power line frequency (e.g. typically 50 Hz or 60 Hz). The signal is measured at the receiver as a function of time. The signal amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, yields the conductivity contour maps.
In known TDEM systems, during the current-on-period, weak conductors produce weak dB/dt signals at the receiver coil while good conductors produce large in-phase signals, although quite small compared to the unwanted primary EM field generated by the transmitter coil system. During the current-off-period, weak conductors produce a large dB/dt signal at the receiver coil from a rapidly decaying EM field while good conductors produce small signals from a slowly decaying EM field. Measurements are typically made during the off-period, and while measurement of dB/dt is preferable to map weak conductors, the measurement of the magnetic field, referred to as the B-field, can increase the accuracy of information provided for good conductors.
In known methods the magnetic field B can be obtained either by direct measurement using a magnetometer or by time-integrating the signal dB/dt measured with a receiver coil. When the magnetic field B is to be obtained by integration, the dB/dt response over the full waveform has to be measured including during the on-period, in order to determine the integration constant that provides a zero DC component over the entire period (see Smith, R. S. and Annan A. P., “Using an induction coil sensor to indirectly measure the B-field response in the bandwidth of the transient electromagnet method”, Geophysics, 65, p. 1489-1494).
An HTEM system is provided by Geotech, known generally by the name VTEM (Versatile Time-Domain Electromagnetic). In addition to this system, several other systems are also available from other companies. These include: AeroTEM by Aeroquest Ltd., THEM by THEM Geophysics Inc., HoisTEM by Normandy Exploration Ltd., NewTEM by Newmont Mining Corp., ExploHEM by Anglo American, SkyTEM by SkyTEM ApS., and HeliGEOTEM by Fugro Airborne Survey.
Some of the commercially available geophysical airborne time-domain EM systems can measure the B-field, such as MegaTEM, and HeliGeoTEM. All of these systems are deployed with transmitter and receiver coils that are physically separated by a large distance, typically in the range 30 m to 150 m. Currently, the only commercially available concentric dipole HTEM system, other than VTEM, is AeroTEM. However, this system does not determine the B-field, but merely outputs the dB/dt signal at the receiver coil during part of the transmitter coil on-time, in addition to the standard off-time measurement.
The prior art generally experiences problems affecting the accuracy of existing systems. For example, acquiring data over the entire period in a concentric dipole HTEM system can be challenging since the signal during the on-period is typically many orders of magnitude higher than during the off-period. As the dynamic range is dictated by the ratio of the signal strength at the receiver coil during the on-period and the off-period of the transmitter coil current, one possible solution is to increase the physical separation between the transmitter and the receiver coils. This large distance has the effect of decreasing the requirement for a large dynamic range for the data acquisition system. However, the separation imposed by the large distance introduces negative characteristics such as loss of spatial resolution or a system that is unwieldy and difficult to tow in flight, especially by helicopter.
Another possible solution to the dynamic range issue is to implement a bucking coil to decrease the amplitude of the primary field at the receiver. Existing prior art that uses a bucking coil is generally restricted to EM measurements in the frequency-domain and has not been implemented for measurement in time-domain. Known methods of achieving frequency-domain EM measurements require a mechanically rigidly mounted bucking coil relative to the transmitter coil and the receiver coil in order to minimize the spurious signals at the receiver coil during data acquisition time caused by the changing geometry of the transmitter, and the bucking and receiver coils. The prior art encounters multiple problems such as, for example, structural vibrations of the coils produced during flight by wind buffeting or aircraft machinery, resulting in spurious signals at the receiver coil with a wide frequency spectrum.
What is needed is a system capable of probing mineral deposits at depths approaching one kilometer and detecting good conductors in the ground, that overcomes the flaws inherent in the prior art.